The formation of dioxins in combustion systems

Flame chemistry in incineration systems involves the formation of many organic products of incomplete combustion, including chlorinated species such as polychlorinated biphenyls (PCB), polychlorinated dibenzo-p-dioxins (PCDD), and polychlorinated dibenzofurans (PCDF). Because the latter are of environmental concern, a great deal of research has been expended on understanding their formation. There are two temperature windows in which they can form: the homogeneous route between 500 and 800 degreesC and the heterogeneous one at 200 to 400 degreesC. Homogeneous reactions, which are the result of the pyrolytic rearrangement of chlorinated precursors, such as chlorophenols and chlorobenzenes in the gas phase, have not been researched as extensively as the heterogeneous mechanism. Heterogeneous formation is a catalysed reaction, which takes place on the ash or soot particles present in combustion systems. There are conflicting views regarding the relative amounts of PCDD/F formed from precursors such as chlorophenols in comparison with the de novo process during commercial operations. The de novo reactions involve the oxidation and chlorination of any unburned carbon in the particulates. The reaction pathways for de novo PCDD/F are based on preexisting 3-ring carbon skeletons; single-ring chlorinated precursors are not intermediates. The formation process is driven by oxidation, and the rate is related to carbon burnoff. Dechlorination and decomposition proceed at elevated temperatures. The reaction appears to take place on the global (external) surface of the particles, but is determined by their carbon and chlorine contents. During de novo formation, chlorine is an active agent, either as chloride in the solid phase or as atomic chlorine in the gas. There is always a contribution from solid-phase chlorine, and gas-phase chlorine is active only at higher concentrations when the solid phase is depleted. An excess of chlorine appears to inhibit de novo formation. Different mechanisms are postulated for PCDD and PCDF. Single- and multiring species chlorinate differently, along different paths. The single-ring compounds formed are comparatively loosely held, while the multiring species tend to be strongly held. Much of the PCDD/F formed is retained on the solid surface and is unable to equilibrate with the gas phase under flue gas conditions. De novo catalysis is due mainly to copper, although iron and other metals are active at lower rates. Copper catalyses the oxidation of carbon, as well as the chlorination and dechlorination of organic products. In its two oxidation states it also acts as a shuttle for chlorine between gas and solid. The catalytic effect can be poisoned by sulfur or nitrogen compounds, such as sulfur dioxide and urea. All the formation models proposed to date both for the homo- and heterogeneous routes are inadequate, no doubt as a result of the complexity of the processes. The homogeneous route needs more fundamental research, and as regards the de novo route, more attention needs to be paid to the composition and nature of the ash's surface, including particle size and carbon/catalyst disposition. (C) 2003 The Combustion Institute. Published by Elsevier Inc. All rights reserved.